In an image forming apparatus such as a copying machine or a printer employing an electro photographic process, a toner image formed on a photoconductive drum is transferred onto a transfer member such as a transfer belt. Thereafter, the transferred toner image is melted by a fixing device including a pressing roller, a fixing roller and a fixing belt suspended around the fixing roller, and is fixed on a recording medium such as a sheet.
As a method of heating and melting a toner in the fixing device, for example, an induction heating system (an IH system) for feeding a high-frequency current to an induction coil, causing an induction current (an eddy-current) in a fixing roller or a fixing belt, causing the fixing roller or the fixing belt to generate heat is put to practical use.
In a fixing device of such an IH system, temperature control is performed by changing electric power supplied to the induction coil. The temperature of the fixing belt or the fixing roller is detected, maximum power is supplied until the detected temperature reaches target temperature, and electric power to be supplied is reduced when the temperature reaches the target temperature. Thereafter, in order to keep the temperature of the fixing belt or the fixing roller constant, the electric power to be supplied is switched between a set minimum power and 0 W. Such a control system is disclosed in, for example, JP-A-2002-229377 (Kyocera Mita Corporation).
On the other hand, in the fixing device of the IH system, there is known an external IH fixing system for arranging divided IH coils on the outside of the fixing device. In such a fixing system, the fixing device includes divided coils including a center coil that heats the center in a width direction or an axial direction of a fixing belt or a fixing roller and side coils that heat ends in the width direction or the axial direction of the fixing belt or the fixing roller. In power supply to the divided IH coils, the electric power is temporally alternately supplied in order to save the electric power.
In the case of the fixing device of the IH system including the divided coils, if power on and off control is performed in a state in which at least one of target portions of the fixing belt or the fixing roller heated by the divided IH coils does not reach control temperature, a deficiency that some portions are kept at the control temperature and the other portions are not kept at the control temperature occurs. This causes temperature unevenness in the width direction or the axial direction of the fixing belt and the fixing roller and causes fixing failure.